Brushless DC motors/generators

ABSTRACT

In a rotational/linear brushless DC motor/generator the rotor/slide has many equally spaced permanent magnet poles of alternating polarity and it has no or few magnetically permeable iron parts. These magnets cover typically slightly more than half the pole pitch in trapezoidal embodiments. The stator consists of one or several pairs of two stator parts facing each other. Each stator part has a plurality of poles with the same pitch as the rotor/slide poles. The stator parts in a pair are arranged on each side of the rotor/slide and are displaced 180 electrical degrees from each other. Each one of the two stator parts in a pair has windings for one phase in order to polarize the poles in the stator part in alternating polarity. The gaps between the stator poles facing the rotor/slide in the same stator part are small compared to the pole pitch. In this way high torque/force DC motors/generators with low weight and high efficiency are provided.

FIELD OF THE INVENTION

The invention is concerned with two-phase or section wise twophasepermanent magnetized, brushless DC motors and generators, rotating orlinear.

BACKGROUND OF THE INVENTION

Most electric motors can equally well be used as a motor or as agenerator. In the following the word motor is generally used even if alldescribed embodiments equally well can be used as generators.

Many electric motors or generators can be built either as rotating orlinear machines with little changes. In the following it should beunderstood that even if some embodiments are described only forrotational or linear machines, the same principles can be applied forthe other kind of electric machine. Thus for instance the words "slide"or "slider" and "rotor" may signify basically the same part.

The market for brushless DC motors in the range of several Watts andhigher is totally dominated by designs having stators where the windingsof three phases are overlapping. This means that the area circumventedby a coil belonging to one phase do not only contain a flux carryingiron pole; it will also circumvent slots containing windings belongingto other phases. This gives a not very efficient use of pole iron massand/or requires long copper windings.

The market for brushless DC motors in the range of several Watts andhigher is also totally dominated by designs having rotors where thepermanent magnets are assembled on a magnetic flux permeable supportstructure (normally made of iron) that closes the flux path. Thisstructure gives a high inertia or mass of the rotor.

A typical prior art motor is disclosed in the international applicationPCT/DE86/00437. This motor has two stator parts and both stator partscarry windings of more than one phase. This means that the areacircumvented by a coil belonging to one phase do not only contain a fluxcarrying iron pole; it will also circumvent slots containing windingsbelonging to other phases. This gives an inefficient use of the poleiron mass and requires long copper windings, that is this prior motor isheavy and has high copper losses. Furthermore, to use the multi-phasewindings efficiently, the magnets must cover a large part of thecircumference, thus giving rotors with a high moment of inertia.

Motors where each stator is divided into two parts offer increasedpossibilities for simplified windings and thus for low cost motors.There are several prior art designs using a single coil windingconcentric with the motor axis for each pase. However, they have severelimitations when used for highly efficient low inertia motors.

One such prior art motor is disclosed in U.S. Pat. No. 4,714,853. Herean annular permanent magnet rotor, which has a basically cylindricalshape, is inserted between two stator parts, each carrying one phase.Each stator is made of two mating, cuplike pole pieces of a single steelplate defining an annular closed space where the windings are located.The pole pieces have a plurality of salient poles facing the rotor. Theradial thickness of the teeth is constant, and there is a considerablegap between the salient poles from each pole piece. If used with smallmotors using weak magnets (the preferred embodiment in FIG. 2 of U.S.Pat. No. 4,714,853 evidently uses ferrite magnets), the total fluxgenerated will easily pass through a steel sheet of acceptablethickness. If run with low speeds, the flux change frequency will below, thus giving acceptable eddy current losses in the pole pieces. Theeddy currents will however increase rapidly for larger motors and higherenergy permanent magnet material. Both of these cases will require anincreased pole piece thickness, which will rapidly increase the eddycurrents at a given speed. Higher speeds will also increase the eddycurrents due to higher flux change rates. If the tangential gap betweenthe salient poles is kept small, the self-inductance of the phasewinding and the portion of the rotor flux lost due to stray flux betweenthe salient teeth will be considerable and will increase rapidly withincreasing radial thickness of the teeth (i.e. due to thicker sheetmaterial in the pole piece), increased stator height and increasednumber of teeth. If the tangential gap is large compared to the polepitch, the residual torque will be high. The design will also carrystray fields outside the motor in some positions of the rotor due to thesame effect as will be described below for FIG. 4. The effect is howeverprobably small due to the limited flux which can be carried in thesingle sheet tooth.

The U.S. Pat. No. 4,922,145 and the Japanese patent publication 62-95958are both concerned with two-phase motors having permanent disc-shapedrotors having magnets covering the total circumference. Both designshave two stators, each one built from two parts and having a singlephase coil winding in each stator. If attempted for high performancemotors, both designs would face the same problems as indicated above forU.S. Pat. No. 4,714,853. Both motors have a constant axial thickness ofmajor part of the salient poles, this giving the same problems asdiscussed above for the constant radial thickness of the cylinder statorof U.S. Pat. No. 4,714,853.

Both designs will cause stray fields outside the motor in some positionsof the rotor due to the same effect as will be described below for FIG.5.

Prior art high performance motors using two stator parts, each onecarrying a single-phase winding, avoid the problems discussed above bythe use of different mechanical arrangements of the flux carrying statorparts which face the permanent magnet rotor. An embodiment of such amotor is disclosed in EP 0 319 632 A1. In this disc rotor motor, the twostators work on different segments of the disc rotor. This reduces theproblems of stray flux losses between the salient poles, as the rotorfacing stator poles of phase 1 are grouped together in one group and allrotor facing poles of phase 2 are far away. This arrangement willhowever only engage about 40% of the disc rotor magnet poles. In theactive area of one stator part, only every second disc rotor pole willbe engaged. There are also parts of the rotor which do not engage anystator pole at all, as the stator windings occupy some of the space thatshould contain stator pole pieces. This will require the use of a highgrade permanent magnet material which can maintain its magnetic strengtheven in an open magnetic circuit at high temperatures.

OBJECTS OF THE INVENTION

It is a primary object of the present invention to obtain electricmotors with a high torque/force to weight ratio and high efficiencyhaving a reasonably low cogging torque/force.

It is another object of the invention to permit a highly efficient useof low cost grades of high energy magnets.

DESCRIPTION OF THE INVENTION

The motor or generator according to the invention solves or at leastsignificantly reduces the problems mentioned above encountered in priorart motors. Also many of the drawbacks mentioned are eliminated in themotors or generators according to the invention as defined in theaccompanying claims.

The permanent magnet rotor/slide has many equally spaced poles ofalternating polarity having no or little magnetically permeable ironparts. The permanent magnet rotor/slide magnets cover typically slightlymore than half the pole pitch.

The stator consists of one or several pairs of two stator parts facingeach other. Each stator part has a plurality of poles with the samepitch as the rotor/slide poles. The stator parts in a pair are arrangedon each side of the rotor/slide, and are displaced 90 electrical degreesfrom each other. Each one of the two stator parts in a pair has windingsfor one phase in order to polarize the poles in the stator part inalternating polarity. In some embodiments, several pairs of statorsparts act on different part of the same rotor/slide.

The gaps between the stator poles facing the rotor/slide in the samestator part are small compared to the pole pitch. The flux conductingparts of the stator are in some embodiments made of plastic mouldedparts formed of iron powder bound together by a thermoplastic materialwhich covers the particles and which serves to insulate the particlesfrom each other.

In most permanent magnet synchronous motors, the permanent magnets coverthe whole circumference of the rotor. The motors according to thepresent invention can be designed to correspond to either trapezoidal orsinusoidal motors. In trapezoidal rotary motors according to the presentinvention, the permanent magnets may in most cases cover only about 55to 70% of the circumferential length of the rotor. In the case of atrapezoidal linear motor this corresponds to about 55-70% of the activelength of the slide or moving part. This reduces the inertia of therotor or mass of a linear motor slide, and will also reduce the totalflux from the rotor. This will give lower fluxes in the stator. Thispermits either a lower flux density in the stator soft iron part, givinglower iron losses, or thinner soft iron parts, thus giving more spacefor windings and lower copper losses. For sinusoidally magnetized motorsaccording to the present invention the rotors and slides must havepermanent magnet material along almost the total circumference orlength. In some embodiments, for example in small motors, it may be costadvantageous to make for example a rotor of a homogenous disc even fortrapezoidal motors, as this reduces the assembly cost. Such discs willthen be given poles by selective magnetization of parts of the disc.

Thus in one aspect of the invention a brushless, electric motor orgenerator is provided, comprising an annular rotor having the shape of arelatively thin ring, such as a cylinder or a flat ring. The rotor isarranged to rotate around a rotation axis and is provided with permanentmagnet poles, which are alternatingly polarized around the rotor. Themotor also comprises an inner stator and an outer stator comprising apermeable base ring provided with permeable salient poles, these poleshaving coils wound thereabout. The coils belonging to all of the polesof one stator are connected to form one electrical phase and the coilsbelonging to all of the poles of the other stator are connected to formanother electrical phase.

The stator poles may have a width perpendicular to the direction ofmovement of the rotor, which is approximately uniform over the length ofthe poles, at least in the regions adjacent to the rotor.

The stator poles may comprise a first portion located at said base ring.This first portion of each pole may then have sides, which are locatedsubstantially perpendicular to the direction of movement of the rotorand also to the surface of the rotor adjacent to the pole. The statorpoles may comprise lateral protrusions at their ends facing the rotor,the protrusions being connected to the first portion and extendingsubstantially in a direction in parallel and antiparallel with thedirection of movement of the rotor, in such a way that only a small gapin the movement direction of the rotor is left between adjacent poles ofthe inner stator and also of the outer stator compared to the extensionof the poles at their sides facing the rotor and in the movementdirection of the rotor.

The length of the each one of the lateral protrusions of a pole in thedirection in parallel and antiparallel with the movement direction ofthe rotor may then be of the same order of magnitude as the length ofthe first portion of the pole in the same direction, for instance abouthalf of the first portion length.

Only a part of the rotor circumference may be provided with permanentmagnet poles. Thus a substantial part of the rotor circumference will beuncovered with magnet poles, in such a way that the permanent magnetpoles for instance only cover somewhat more than the half, as 55-70% ofthe circumference of the rotor.

In another aspect of the invention a motor a generator is providedhaving an annular rotor arranged to rotate about a rotor axis and havingthe shape of a thin cylinder and provided with permanent magnet poles,which are alternatingly polarized around the rotor. The motor orgenerator also has an annular inner stator and an annular outer stator,each one comprising a pair of pole pieces. Each one of the pole piecesis also annular and the pole pieces of each pair are located in opposedrelationship with each other. Each pole piece comprises on the peripherythereof which faces the rotor circumferentially spaced salient statorpoles in such a way that the poles of one pole piece of a pair isinterleaved with the poles of the other pole piece of the same pair.Each one of the stators carries an annular coil concentric with therotation axis, where the coil of the inner stator is connected to formone electrical phase and the coil of the outer stator is connected toform another electrical phase. The pole pieces of each pair form anannular groove, which for the inner stator is open to the rotation axisand for the outer stator is open in the opposite radial direction. Eachone of said grooves carries one of said annular coils and a permeablering is located on the open top of each one of said grooves.

The permeable ring may comprise a large number of metal sheet elementshaving their sides radially aligned and secured to an inner and outerretaining ring for the inner and outer stator, respectively.

In another aspect of the invention a motor or generator is provided,comprising an annular rotor arranged to rotate about a motor axis andhaving the shape of a thin cylinder and provided with permanent magnetpoles, which are alternatingly polarized around the rotor. The motor orgenerator also comprises an annular inner stator and an annular outerstator, each one comprising a pair of pole pieces. Each one of the polepieces is also annular and the pole pieces of each pair is located inopposed relationship with each other. Each pole piece comprises on theperiphery thereof which faces the rotor circumferentially spaced salientstator poles, the poles of one pole piece of a pair being interleavedwith the poles of the other pole piece of the same pair. Each one of thestators carries an annular coil concentric with the rotation axis, thecoil of the inner stator being connected to form one electrical phaseand the coil of the outer stator being connected to form anotherelectrical phase. The salient poles of the pole pieces extend generallyradially and the salient poles are provided with lateral protrusions attheir ends facing the rotor in such a way that only a small gap is leftin the circumferential direction between said protrusions of adjacentpoles of the inner stator and also of the outer stator.

The ring part of each pole piece may be substantially flat and locatedin a radial plane. The inner portions of the salient poles may extendgenerally in parallel with the ring part and the outer portions of thesalient poles facing the rotor may extend at an oblique angle inrelation to plane of the inner portions and the substantially flat ringpart, this angle preferably comprising about 45°, e.g. being in therange 15°-75°.

The pole pieces are preferably made of laminated iron, comprising sheetelements having a configuration including a flat ring part and obliquelybent, salient parts. Also sheet elements having a shape onlycorresponding to said ring part may be included. These latter elementsare then interposed between the first mentioned elements.

In another aspect of the invention a motor or generator is provided,comprising a moveable thin part having a constant width and providedwith permanent magnet poles, which are located on the opposite largesurfaces of the moveable part and are alternatingly polarized. The motoror generator also comprises a first stator part and a second statorpart, each one comprising a pair of pole pieces. The pole pieces of eachpair are located in opposed relationship with each other and each polepiece comprises on the surface thereof, which faces the large surfacesof the moveable part, spaced salient stator poles, in such a way thatthe poles of one pole piece of a pair is interleaved with the poles ofthe other pole piece of the same pair. Each one of the stator partscarries a coil having a large portion of its wound parts locatedsubstantially in parallel with the large surfaces of the thin moveablepart, the coil of the first stator being connected to form oneelectrical phase and the coil of the second stator being connected toform another electrical phase. Only a part of the large surfaces of themoveable part is provided with permanent magnet poles in such a way thepermanent magnet poles preferably only cover 55-70% of the circumferenceof the rotor.

The salient poles of the pole pieces may extend generally in planesperpendicular to the large surfaces of the moveable part. The salientpoles may be provided with lateral protrusions at their ends facing themoveable part, the protrusions being directed in the generallylongitudinal direction of the moveable part in such a way that only asmall gap is left in the longitudinal direction of the moveable partbetween said protrusion of adjacent poles of the first stator and alsoof the second stator.

The outer portions of the salient poles facing the moveable part mayextend at an oblique angle in relation to the large surfaces of themoveable part, this angle preferably comprising about 45° or being inthe range of 15°-75°.

A base part of each pole piece which is located opposite to the moveablepart may be located generally perpendicularly to the large surfaces ofthe moveable part. The inner portions of the salient poles may thenextend generally in parallel with the base part and the outer portionsof the salient poles facing the moveable part may extend at an obliqueangle in relation to plane of the inner portions and the substantiallyflat base part, this angle preferably comprising about 45° or being inthe range of 15°-75°.

The first stator and/or the second stator may comprise two substantiallyequal stator segments, each stator segment in turn comprising the twopole pieces. These stator segments for each stator may then be locatedin parallel with each other and carry winding portions belonging to thesame coil, these winding portions being generally in parallel with thelongitudinal direction of the moveable part.

In another aspect of the invention a part of an electrical machine,typically a rotor or a slide, is provided, comprising permanent magnetsattached to a base, said part being movable in relation to another partof the electrical machine, typically the stator. Said magnets aremagnetized in a direction substantially perpendicular to the directionof movement and they may, as is conventional, be distributedsubstantially uniformly over at least a part of the base. The area ofthe cross sections of a permanent magnet perpendicularly to thedirection of movement varies in said movement direction in such a waythat said area is significantly larger in a cross section locatedcentrally in said permanent magnet than in cross sections located moreclose to an edge of said magnet.

Said area of cross sections may then decrease monotonically, at leastover a significant distance, in the direction from the centre of saidmagnet to each one of its edges.

The variation of said area of cross sections may advantageously besubstantially sinusoidal.

A permanent magnet may, as is conventional, then normally have aconstant width as measured perpendicularly to said movement direction.

A permanent magnet may also have a substantially flat base surface,where it is supported by said base.

A permanent magnet may then be homogenously magnetized or preferablyfully magnetized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a sectional view of an embodiment of a motor according tothe invention that has a bellshaped rotor and individually wound poles.The embodiment is suitable for manufacture by stacked iron sheettechnology.

FIG. 1b shows in a larger scale a segment of the airgaps and windings ofthe embodiment of the motor as shown in FIG. 1a, where the principalmagnetic field in the motor segment is illustrated.

FIG. 2 shows a sixth of a six phase motor according to the inventionthat has a bellshaped rotor, individually wound poles and six pairs ofstator parts.

FIGS. 3a and 3b show a the stator and unwound rotor of a motor accordingto the invention that has a bellshaped rotor and individually woundpoles.

FIG. 3c shows a rotor arrangement giving a sinusoidal back-emf usingfully magnetized permanent magnets on the rotor.

FIG. 4a shows an embodiment of a motor according to the invention havinga bellshaped rotor and a single coil winding common to all poles in aphase. The embodiment can suitably be made by the use of stacked ironsheet.

FIGS. 4b and 4c show in plan and elevational views respectively apunched and bent electrical steel plate part used in the inner stator ofthe motor as shown in FIG. 4a.

FIG. 4d shows two punched and bent electrical steel plate parts similarto that shown in FIGS. 4b-4c having a spacer washer between the parts.

FIGS. 4e and 4f show an alternative flux bridge arrangement comprisingan annular ring of electrical steel strips to be used in the stators ofa motor similar to that shown in FIG. 4a.

FIG. 4g is a magnified view of the section indicated in FIG. 4d.

FIG. 5a shows an embodiment of a motor according to the invention whichhas a disc-shaped rotor and a single coil winding common to all poles ina phase. The embodiment can suitably be manufactured by the use of metalpowder technology.

FIG. 5b shows in a large scale the position of the poles and permanentmagnets of the motor as shown in FIG. 5a.

FIG. 5c shows an enlarged view of the pole pieces illustrated in FIG. 5aindicating the main flux leakage region.

FIG. 6a schematically shows a cross section of a linear motor accordingto the invention which has moulded stator parts and a single coilwinding common to all poles in a phase.

FIG. 6b shows a partial longitudinal section of the linear motor of FIG.6a.

FIG. 6c shows a partial top view of the linear motor of FIG. 6a. Themotor of FIGS. 6a-6c is suitably made by the use of stacked iron sheetor metal powder technology.

FIG. 6d shows schematically, in a view similar to that of FIG. 6a, aportion of a stator made of stacked electrical steel sheets for a linearmotor similar to that in FIGS. 6a-6c.

FIG. 7a shows an embodiment of a motor according to the invention whichhas a bellshaped rotor and a single coil winding common to all poles ina phase. This embodiment is suitably manufactured by the use of metalpowder technology.

FIGS. 7b-7d show sections of a moulded part constituting one fourth ofthe magnetically permeable parts of the stator system of the motor asshown in FIG. 7a along the lines B--B, C--C and D--D respectively.

FIGS. 8a-8c show an embodiment of a motor according to invention whichhas a discshaped rotor and stator parts built of totally four identicalmoulded iron powder parts Principally the same motor can be built usingstacks of flat electric steel parts of two to three differently punchedshapes.

FIG. 9a shows an embodiment of a motor according to invention which hasa beltwheel shaped rotor and two stator parts for each phase, each phasehaving a single coil winding. The embodiment is well suited to bemanufactured by means of metal powder technology.

FIGS. 9b-9c show an embodiment of a motor according to invention whichhas a beltwheel shaped rotor, four stator parts and four phases. Theembodiment is well suited to be manufactured by rolled steel.

FIG. 10 shows a four phase linear motor according to the inventionhaving individually wound poles and two pairs of stator parts.

FIG. 11 shows a winding arrangement for a motor similar to that shown inFIG. 10 but intended for motors having far more poles in each group.

FIGS. 12a and 12b show a principal flux pattern in respectively aconventional brushless DC motor and the motors according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter the word "rotor" is used to indicate the conventionallymoving part of an electric rotational motor and the word "stator" forthe conventionally stationary part. However, in some applications, thepart named "rotor" may be the stationary part and the part "stator" themoveable part. It should be understood that these words are only usedfor the ease of describing the invention and that the invention could beused equally well for motors having a stationary "rotor" and a rotating"stator".

In the same way, the words "slide" or "slider" and "stator" are used inthe way conventional for linear electric motors. It should thus also beunderstood that these words are only used for the ease of describing theinvention and that the invention could be used equally well for motorshaving a stationary "slide" or "slider" and a moving "stator".

Also, many of the embodiments described only for rotational electricmotors can easily be adapted for use as linear motors and vice versa.Thus the invention must be considered to apply to all electric motors.Also, linear motors can have a curved shape of the path of the moveablepart.

FIGS. 1a-1b show a first embodiment of a rotary motor according to theinvention. FIG. 1a is a sectional view of the motor as seen in adirection perpendicular to its axis of rotation and FIG. 1b shows anenlarged portion of the same view.

The motor comprises an annular rotor 100, which is inserted in acylindrical slot between two stators, an inner stator and an outerstator. The active parts of the rotor preferably comprise separatemagnets 101, in the case shown 16 magnets; the rotor could howeverinstead comprise a partly magnetized annular ring. The separate magnetsare bound to each other or supported by a support element by amagnetically inactive material which is not relevant for the inventionand they will thus form a bellshaped rotor. The magnet poles maypreferably be made of permanent magnet material like NdFeB or NdPrFeB orSnCo. The permanent magnet poles are oriented so that all even numberedpoles have the same polarity and all odd numbered poles have theopposite polarity. The magnets cover somewhat more than 1/2 of the airgap circumference, for instance about 55-70%.

The inner stator comprises a base ring 110 and 16 pole teeth 111-126extending radially from the motor axis and made of for example laminatediron like in most conventional electric motors. In the enlarged view inFIG. 1b, the pole tooth 117 is completely visible.

All stator poles (like 111-126) of the inner stator have windings. Onlythe winding 127 around pole 117 is shown in FIGS. 1a and 1b. Theembodiment as shown in FIGS. 1a and 1b is a 16 pole motor (i.e. therotor has 16 poles) with two electric phases, each having or belongingto one pole group; the 16 poles 111-126 on the inner stator form onegroup and the 16 poles 131-146 on the outer stator form another group.

All stator poles in one group have the same angular pitch (in the caseof the embodiment of FIGS. 1a and 1b 360/16=22.5 degrees), which is thesame as the pitch of the permanent magnet poles on the rotor. The gapsbetween the portions of the stator poles immediately facing the rotor inthe same stator part are small compared to the pole pitch, for instanceabout 1/10-1/20 of the pole pitch. Therefore the poles havecircumferentially protruding parts, that is parts which extend along andopposite the path of the moving part, these protruding parts having thelargest extension in the region immediately adjacent to the surface ofthe stator and facing the path of the moveable part.

The wound poles have a coil wound around each single pole. The sectionsthrough the 13 wire turns constituting the winding 123 around the pole117, as seen in FIG. 1b, are marked with dots on the left side andcrosses on the right side of the pole. The coils around the evennumbered poles 112, 114, 116, 118, 120, etc. are wound in the samedirection while the coils around the odd numbered poles 111, 113, 115,117, 119, etc. are wound on connected in the other, opposite direction.When the coils are energized by current from the associated powerelectronics, this current will in the inner stator magnetize the oddnumbered poles 111-125 in one direction and the even numbered poles112-126 in the other or opposite direction.

The outer stator has a similar configuration as the inner stator with abase ring 130 and 16 radially salient poles 131-146 with laterallyextending portions. The winding pattern is the same; all odd numberedpoles are wound or connected to give each pole a polarity opposite tothat of the two adjacent poles. The two stators are securedconcentrically to each other and 90 electrical degrees (in the caseshown 11.25 mechanical degrees) apart.

FIG. 1b also illustrates the magnetic field or the flux flow in therotor and the two stators. In order to facilitate the understanding ofthe Figure, each permanent magnet 101 is assumed to be able to drive amagnetic flux corresponding to six flux lines in FIG. 1b. As is shown inthe Figure, the flux density can be kept fairly balanced both over thesurface cf the permanent magnets, in the poles and in the base rings 100and 120. This permits a thin stator with a uniform and well utilizedflux conducting material. This will thus allow light motors generatinghigh mechanical torques.

The relatively constant flux density for all permanent magnets will keepthe lowest flux density of the permanent magnets high. This will, withpresent magnet technology, permit the use of cheaper magnet materials(like NdFeB instead of SmCo) or grades with higher energy density (likeusing NdFeB materials with BH products of about 260-335 kJ/m3 instead ofmaterials with about 210-265 kJ/m3).

As is easily understood from FIG. 1b, a clockwise movement of the rotorpermanent magnets will, during the first almost 10 mechanical orgeometrical degrees, hardly change the magnetic flux through the sectionof the pole 136, which is indicated at 143--143 in FIG. 1b. Thereforealmost no emf (electromotive force) is induced in the coil wound aroundthe pole 136 (and because of the symmetry, neither in any other coilwound around the outer stator poles 131-142). On the contrary, therewill be a large change in the flux through the coil 123 wound around thepole 117 (and because of the symmetry, also in the coils wound aroundall inner stator poles 111-122).

If the clockwise movement discussed above should continue approximately9 mechanical degrees, almost all magnetic flux from the magnet 103 willhave left the pole 116 and been transferred to the pole 117. In this newposition some of its magnetic flux will start to pass by the pole 137.During the first 9 degrees of the rotor movement, practically the wholeflux from the pole 103 was directed through the pole 136. Therefore,when the flux change through the poles 111-122 has almost ended, thehereto almost static magnetic flux through the poles 131-142 will startto change. In this way, in any position of the rotor, current througheither the inner or the outer stator will be able to create a torque onthe rotor. The other stator may act as a passive flux path and thecurrent through its windings should preferably be zero. (In somepositions, i.e. if the rotor in FIG. 1b were to be moved 1 degreecounterclockwise, both stators could have one half of the normalcurrent.)

The embodiment shown thus can always use the full strength of all thepermanent magnets. (The maximum short term torque is limited by theability of the permanent magnets to force a magnetic flux through thewinding in spite of the winding current.) The motors according to theinvention thus do not waste expensive permanent magnets by having partsof them partially utilized or completely unutilized.

FIG. 2 shows a sixth of a six phase rotation motor according to theinvention having individually wound poles.

The rotor has 40 poles, of which nine 201-209 are shown.

There are twelve stator parts, two for each phase. The inner stator hassix pole groups, of which only one is completely shown in the Figure andis located between the dotted radial lines. The group has six poles221-226. The pole pitch for the six poles in any pole group is identicalto or very close to the pitch of the rotor magnets. Adjacent pole groupsare separated by unwound flux balancing poles 292 and 294.

The outer stator has six pole groups, of which only one is completelyshown in the Figure. The group has six poles 251-256. The pole pitch forthe six poles in any pole group is identical to or very close to thepitch of the rotor magnets. Adjacent pole groups are separated bypreferably unwound extra, flux balancing poles 291 and 293. The windingson the outer stator are not shown.

Each inner stator group is connected to the same phase as the opposite,diametrically located inner stator group, thus giving three phases onthe inner stator.

Each outer stator group are in phase with the diametrically oppositeouter stator group, thus giving three phases on the outer stator. Thetotal assembly gives six phases. If the motor magnets and windings areto be fully utilized, these can be driven individually, for example byusing one H-bridge for each phase. To optimally utilize the magnets, thetwo stator pole groups shown in the Figure could be connected to the twophases of a two-phase system (as they face the two sides of the samepermanent magnets). The three two phase systems will then be 30electrical degrees apart or offset. The arrangement gives a mechanicaltorque having a smaller torque ripple than in the simple motor as shownin FIGS. 1a and 1b. Also the cogging torque will be lower, as for every30 electrical degrees some rotor magnets will face the center of two ofthe twelve pole groups.

FIGS. 3a and 3b show an axial motor according to the invention. Thestator is preferably made of rolled electrical steel. Equipment forproducing stator laminations by coiling endless electrical strip ironand stamping the winding slots with gradually increasing slot-to-slotdistance to compensate for the successively increased diameter arecommercially available.

FIGS. 3a and 3b show the unwound stator and rotor. There are two statorparts 301 and 302, each having 8 poles like 303 kept together by a basering 304. Each pole has a winding (not shown), and all windings on thesame stator part are connected to the same phase.

The rotor 304 has the form of a continuous ring with sinusoidalmagnetization and 8 poles. The inner and outer radii of the ring areconstant, but the intensity of the magnetization is different ondifferent parts on the ring. This is illustrated by different sizes ofthe N and S symbols on the ring in FIG. 3a.

FIG. 3b shows the unwound stator part 301 as seen from the rotor. InFIG. 3a the width relation between the length 305 of the first pole partclose to the base ring 301 and the added length 306 of the pole pieces308 protruding from this basic pole width 305 is much less than 2, whichis a typical relation as seen in the other figures like FIG. 1, 2 and10. This is a consequence of the stator shape. As can be seen in FIG.3b, the same relation in the inner side of the stator, i.e. the relationbetween the pole width 309 facing the rotor and the pole width 310 ofthe pole from the base ring is approximatively 2 to 1 as in the otherFigures.

The sinusoidal magnetization of the rotor does not require any changefrom the pole width facing the rotor and the pole width close to thestator base ring. The total flux from a sinusoidally magnetized pole ofwidth 1, length 1 and peak flux density of 1.1 T is the same as for amagnet with width 0.6366, length 1 and flux density of 1.1 T. As thepermanent magnet poles for trapeziodally, magnetized motors according tothe invention normally is assumed to cover some 55-70% of the rotorsurface, the total flux generated by such rotors is close to that of aideal sinusoidal rotor that covers the whole rotor surface, and would beidentical if the trapeziodally magnetized motor had an ideal magnetcovering 63,66% of the surface. The relation 2 to 1 between the polewidth 309 facing the rotor and the pole width 310 of the pole from thebase ring is a consequence of available materials. A pole front width(309) of 1 cm and length 1 cm will receive a peak flux ofapproximatively 1.1 T×0.636=0.70E-4 Vs. To keep the peak flux density tosome 1.5 T in the base pole iron, its width (310) should be in the orderof 0.7/1.5=0.46 cm, giving a relation of approximatively 2 to 1.

FIG. 3c shows a rotor where a approximatively sinusoidal back emf isarranged by the physical shape of the permanent magnets. Unlike theconventional arrangement described for FIG. 3a, the magnets are fullymagnetized over their full surface. This arrangement permits the use offully magnetized magnet material, which reduces magnet materialconsumption and follows the design goal of the invention to offer astator design where lower cost high energy magnets always can maintain ahigh flux intensity on their surface.

In the FIG. 3c, this is arranged by a magnet shape where one singlepiece 310 has the approximative form of half a sine wave. The magnetslike 310 may be glued to an 8 corner disc 311 with an axis 312 and maybe additionally secured by the winding of a thin filament element suchas high tensile strength wire 313 around the magnets.

The same inventive idea can be embodied in linear slide motors accordingto the invention and in cylinder shaped or bellshaped rotor motorsaccording to the invention. It can also be applied in conventionalbrushless motors.

FIG. 4a shows another embodiment of a rotary electric motor according tothe invention. Instead of having one coil wound around each pole likethe coil 123 around pole 117 as shown in FIGS. 1a-1b, the winding foreach phase comprises one single coil around the rotor shaft of themotor.

The magnetically permeably parts of the inner and outer stators can bemade of laminated, rolled electrical steel in the same way as in mostconventional electric motors as is illustrated in the Figure, but theycan alternatively each be formed by two moulded parts comprising acomposite material of for example surface coated iron powder embedded ina plastic binding phase as will be discussed below with reference toFIGS. 7a-7d.

The FIG. 4a shows a section through the motor and FIGS. 4b and 4c showone of the punched and bent parts made of rolled electrical steel plateand forming the upper half of the inner stator.

The motor has twelve poles. More poles will increase the torque for agiven small coil current, but it will increase the stray flux for highercoil currents and increase the iron losses for a given speed.

The inner stator comprises a lower part 402, a magnetically permeableannular element 401 and an upper part 403 consisting of the sameelements as the lower part 402. Each of these parts 402 and 403comprises a number of magnetically permeable laminated sheets. One suchelement is shown in FIGS. 4b and 4c. The annular element 401 cancomprise a roll of steel sheet like in a toroid transformer, a stack ofpunched rings or a moulded part comprising small iron particles.

The inner stator winding 451 has the form of a simple coil concentricwith the motor axis 407.

FIG. 4b shows one of the elements constituting the upper and lowerstator parts 402 and 403. The even numbered poles 412, 414, etc. of thiselement are produced by first punching a sheet and then bending thepoles as shown in FIG. 4c. The poles 412, 414, etc, of the upper statorparts shown in FIG. 4b correspond to the poles 112, 114, etc. of FIG. 1.The gaps between the stator poles facing the rotor in the same statorpart are small compared to the pole pitch because of the laterallyprotruding, triangularly shaped parts 408 which extend from theessentially rod-shaped pole stems. (Most of the void between the teethlateral parts indicated at 408 in FIG. 4b will be filled with the oddnumbered teeth belonging to the other half of the inner stator.) Thepurpose of the tooth design is described in detail for the embodimentshown in FIGS. 5a-5c.l

If the bent parts have a uniform thickness, a washer 460 made ofelectrical steel plate can be inserted between each part as shown inFIG. 4d to fill the empty space that would otherwise appear between theparts. To facilitate the understanding of the drawing, only two parts460 have been drawn. Like in most motors, there are normally tens orhundreds of similar parts 460 in each stator. The net magnetic fluxcaused by the rotor magnets in the even numbered poles 412, 414, etc.will have the same polarity and will pass through the base ring 410. Itwill then continue through the annular part 401, the lower inner statorpart 402 and continue to the other rotor magnets through the oddnumbered poles. In FIGS. 1a-1b all net magnetic flux in the pole 117must pass through the coils 123 and one of the coils around poles 116 or118. In the embodiment of FIG. 4a, the net flux through all the six oddnumbered poles must pass the annular part 401 and thus through the innerstator coil 451.

The outer stator is built in the same way as the inner stator. Itsannular part 404 corresponds to the inner stator part 401 and the outerstator coil 453 corresponds to the inner stator coil 451. When the outerstator is built from stacked sheets, these will have the shape of anouter base ring from which the poles (corresponding to every other poleof the complete outer stator) extend inwards in a generally radialdirection. In the embodiment shown in FIG. 4a, the averagecircumferential length of the outer coil is approximately twice that ofthe inner coil. To obtain the same resistance for both phases, the crosssection area of the outer coil should be approximately twice that of theinner coil, which also can be seen as indicated in FIG. 4a.

FIGS. 4e and 4f show an alternative way of arranging the flux path fromthe upper stator part 406 to the lower part 405. It comprises smallvertical strips 462 of rolled electrical steel plate facing the end ofthe horizontal part forming the stator parts 406 and 405. The strips canbe kept in place by a retaining ring 461. With this arrangement the fluxlines in the top sheet part of the stator 406 do not have to pass allthe air gaps between all the other sheet parts to in 406 to reach downto the lower stator part 405.

The current changes in the windings 451 and 453 will impose an emf, thatcould cause a loop current through the shaft, the bearings, the upperbearing fixture 455, the rear cover 454, the outer stator 404-405-406and the front shield 457. To avoid this an electrical insulation shouldbe inserted somewhere in this possible loop, for example by making thebearing fixture 455 of an electrically insulating material.

The permanent magnets 411 of the rotor are inserted in or attached tothe rotor bell 413. The bell 413 could advantageously be made of alight, electrically and magnetically not conducting material. The way ofsecuring of the bell 413 to the rotor shaft 407 is not relevant to theinvention.

The magnet part of the rotor can alternatively be made of a mechanicallyhomogenous ring magnetized to give approximately the same flux patternas individual magnets. Alternatively the magnet part of the rotor can bemade of a mechanically homogenous ring magnetized to give a sinusoidalflux distribution along the rotor with a number of sinusoidal periodsequal to half the number of poles in a the corresponding separate magnetmotor. This will increase the moment of inertia of the rotor but willreduce the cogging torque and give less torque ripple.

FIGS. 5a-5c illustrate yet another embodiment of a rotation motoraccording to the invention. In this embodiment the rotor is shaped likea disk. Unlike the axial motor shown in FIG. 3a, it has one common coilfor each phase. The permanent magnets are inserted in the disk in a waybasically similar to the other embodiments as is shown in detail in FIG.5b, which shows a section along the line b--b in FIG. 5a.

There are two stators, one upper stator, as viewed in the Figure,comprising the moulded magnetically permeable parts 501, 503 and 505,and one lower stator comprising the parts 502, 504 and 506. There is onewinding (507 and 508) in each stator. Each winding has the form ofsimple coil around the rotor shaft 512.

The two stators are located 90 electrical degrees away from each otheras shown in FIG. 5b. The pole pieces facing the rotor have the samebasic shape as in FIG. 1 in the respect that the gaps between the statorpoles facing the rotor in the same stator part are small compared to thepole pitch. This can be seen in FIG. 5b, where the distance between thepoles 505 and 503 is small when seen from the magnets in the rotor disc.This increases the minimum flux density in the rotor magnets, whichpermits a wider selection of permanent magnet materials. It will alsoreduce the residual torque.

The pole pieces are also arranged to reduce the stray flux in the airbetween for example the poles 503 and 505. This is achieved by the shapeas seen in FIGS. 5b-5c. In FIG. 5b it is shown that the distance betweenadjacent pole pieces is short adjacent to the rotor path, where thepoles face the rotor magnets like 511, but is wider further away fromthe rotor magnets. In FIG. 5a and an enlarged detail thereof shown inFIG. 5c is shown that the pole pieces 503 are separated from the polepieces 505 also in the other direction. The main area where the fluxleakage in the air will take place is shaded in FIG. 5c, with moreintense shading in the lower part where the distance between the polepieces 503 and 505 is smaller. In the embodiment shown in FIGS. 5a-5c,this flux leakage area is shaped like a gothic arc. In the motor in FIG.4a it is basically triangular.

The two stators are mechanically attached to each other by means notshown in FIGS. 5a-5c. The three moulded parts like 501, 503 and 505 ofthe upper rotor, as seen in the Figure, are attached to each other byglue, screws or other means not relevant to the invention and not shown.

The permanent magnets 511 of the rotor are inserted in or attached tothe rotor disk, 513. The disk 513 could as above advantageously be madeof a light, electrically and magnetically not conducting material. Theway of securing the disc 513 to the rotor shaft 512 is not relevant tothe invention and is not indicated in the Figure.

Small motors similar to the embodiment shown in FIG. 5a can often withadvantage be made with a homogenous permanent ring replacing theindividual magnets shown in FIG. 5b as this often gives lower assemblycosts, and permits lower cogging torques. A much lower pole number thanthe one shown will give lower iron losses and thus permit operations athigher speeds. It will also reduce the stray fluxes, thus permittinghigher mechanical output power at the expense of higher copper losses atlow speed and moderate torques.

The electric motor according to FIGS. 5a-5c has two potentialdisadvantages. Loop currents caused by current shifts in the windings507 and 508 can flow through the bearings, the rotor shaft and theelements 501-515-502. There are many possible solutions of thispotential problem; one is to make the lower shield 502 of an ironcomposite material having a negligible electrical conductivity. Anotherpotential problem is the stray field which in some rotor positions willflow from the upper stator element 505 to the lower stator element. 506and which could cause undesired effects on objects close to the outersleeve 515 of the motor. This problem can be reduced by increasing thediameter of the outer sleeve 515.

FIGS. 6a-6d illustrate an embodiment of a linear motor according to theinvention. In this embodiment the "rotor" is replaced by a "slider" or"slide" which has two parallel rows of permanent magnets. As in therotary motors described above, the permanent magnets in each row areoriented with opposite polarities.

There are two stator parts, one upper stator, as seen in FIGS. 6a and6c, located on one side of the slide, comprising the mouldedmagnetically permeable parts 601-605, and one lower stator 607 at theother side of the slide. There is one winding (606 and 608,respectively) in each stator. Each winding has the shape of a simplecoil configured as a closed loop having two long, parallel sides.

The two stator are located 90 electrical degrees away from each other asindicated in FIG. 6c. The two stators are mechanically attached to eachother by means not shown in FIGS. 6a-6c. The five moulded parts of eachstator, elements 601-605 of the upper stator, are attached to each otherby glue, screws or other means not relevant to the invention. The polepieces facing the slider have the same basic shape as in FIGS. 1a-1b inthe respect that the gaps between the stator poles facing the slider inthe same stator are small compared to the pole pitch. This can be seenin FIG. 6c, where the distance between the poles 601 and 602 is smallwhen seen from the position of the magnets in the slider. The purpose ofthe tooth design is described in detail for the rotation motor shown inFIGS. 5a-5c.

The slider permanent magnets 611 are inserted in the slider fixture orsupport 612. The fixture 612 could as above advantageously be made of alight, electrically and magnetically not conducting material.

The magnet part of the slider can alternatively be made of amechanically homogenous bar magnetized to give approximately the sameflux pattern as individual magnets. This will however, as above,increase the mass of the slider.

The linear bearings used to keep the slider fixture 612 in place and themeans used to connect the fixture 612 to the load are not relevant tothe invention and are not shown.

FIG. 6d shows the upper stator part of an embodiment similar to the oneshown in FIG. 6a. In FIG. 6d, however, the stator is built of punchedand (for the parts 620 also bent) parts of rolled electrical steelplate. Filler strips 621 may be inserted like the filler washers 460 inFIG. 4d. To facilitate the understanding of the drawing, only two parts620 have been drawn. Like in most conventional electric motors, thereare normally tens of hundreds of similar parts 620 in each stator. Theparts 622 are punched rolled electrical steel parts stacked in a similarway as illustrated in FIG. 4e.

The stator design shown in FIG. 6d can in principle also be used foraxial rotational motors. This requires equipment that like the equipmentmentioned in the description of FIGS. 3a and 3b can punch poles adaptedto the coiling of the stator sheet, but requires the added functionalityof bending the poles after punching.

FIGS. 7a-7d show another embodiment of a rotation motor according to theinvention similar to the embodiment shown in FIGS. 4a-4f. The statorparts are here moulded using small particles of for example iron and aninsulating phase, for example some thermoplastic.

FIG. 7a shows an axial section through the motor and FIGS. 7b-7d showthree radial sections through the lower moulded part, as seen in theFigure, which forms the inner stator. The sections shown in FIGS. 7b-7dare taken along the lines indicated at "B--B", "C--C" and "D--D"respectively in FIG. 7a.

The motor has 16 poles as illustrated in the Figures. It will often besuitable to have many more poles; this will increase the torque for agiven coil current. The higher flux frequency for a given angular speedincreases however with a higher number of poles.

The even numbered poles 712, 714, etc. of the inner stator are mouldedtogether with the lower base ring 717. The moulded parts of the upperand lower inner stator have the same shape but are arranged facing eachother and having an angular offset corresponding to the pole pitch ofthe assembled stator.

The principal magnetic flux paths are the same as in the rotation motorshown in FIGS. 4a-4f. The flux enters all even numbered teeth like 701,passes through the lower base ring 717 and goes to upper inner statorpart through the surface 718. The teeth like 701 are thinner in the topthereof as shown in FIG. 7b and will have a progressively larger crosssection closer to the base ring 717 as shown in FIG. 7c and 7d. Thelarger cross section is needed as progressively more permanent magnetflux has to be transferred through the tooth down to the base 717. Ascan be seen from the section line "D" in FIG. 7a, the section shown inFIG. 7d must carry the total flux from the permanent magnet 719 possiblystanding in front of the tooth. The inner stator winding is one coillocated at 723 and wound around the rotor shaft. The purpose of thetooth design concerning residual torque and flux leakage is described indetail for the embodiment shown in FIGS. 5a-5c.

The permanent magnets 719 of the rotor are inserted in or attached tothe bell-shaped rotor structure 720. The structure 720 may as aboveadvantageously be made of a light, electrically and magnetically notconducting material. The way of securing of the structure 720 to therotor shaft 721 is not relevant to the invention and is not indicationin the Figures. The magnet part of the rotor can, as above,alternatively be made of a mechanically homogenous ring magnetized togive approximately the same flux pattern as individual magnets. Thiswill however, as has been stated above, increase the moment of inertiaof the rotor.

The outer stator is built in the same way. The surface 722 correspondsto the inner stator surface 718 and the outer stator coil located at 724corresponds to the inner stator coil 723. The cross section area of theouter stator coil 724 is larger than that of the inner stator coil 723,thus permitting the resistances to be more equal, as has been alreadydescribed for the motor of FIGS. 4a-4f.

The embodiment according to FIG. 7a has two potential disadvantages.Loop currents caused by current shifts in the windings located at 723and 724 can flow through the bearings, the rotor shaft, the front andrear shields and the outer stator. There are many possible solutions ofthis potential problem; one has been shown as an insulated bearingfixture in FIG. 4a. Another potential problem is the stray field that insome rotor positions will flow from the inner to the outer stator andwhich could cause undesired effects on objects close to the motor frontor rear shields. This problem can be reduced by inserting anelectrically not conducting, heat conducting element 725. This willhowever increase the axial length of the motor.

FIGS. 8a-8c show an embodiment of a motor according to invention whichhas a disc-shaped rotor and stator parts built of totally four identicalmoulded iron powder parts.

FIG. 8a shows a side view of a wound motor with all magneticallyrelevant part in their place. The shaft 801 exits from the front statorpart 802-803, which is built of two poles 802 and 803. The poles consisteach of a moulded iron powder part. This element is shown unwound inFIG. 8c. The two windings 804 and 805 of the front stator part arevisible as well as the winding 806 of one of the two poles of the rearstator part.

FIG. 8b shows a section through the motor of FIG. 8a with the windingwires removed. Only the pole 803 of the front stator parts is seen; thesection extends through the borderline between the two front statorparts 802 and 803. The section view passes through the two rear statorparts 807 and 808. The moulded iron powder part 808 is shown separate asseen in the same sectional view in FIG. 8c.

The motor has a disc shaped rotor 810 consisting of a ring of highenergy magnet material like NeFeB with two poles. Two rotor bearings 811and 812 with nonmagnetic runways enforces an airgap like 814 between therotor 810 and the stator parts; the attraction of the rotor magnet willensure a preloading of the bearings. The distance ring 813 will restrainthe front and rear stator parts from disalignment. To reduce the bearingpreload, a spring washer can be inserted between the stator parts andthe bearings. In such a case, the distance ring 813 will balance theattraction of the rotor magnet on the stator parts. Motor case andauxiliary parts to keep the motor together are not relevant to theinvention and are not shown.

FIG. 9a shows an embodiment of a rotation motor, where the disadvantagesof the above mentioned motors of FIGS. 4a-4f, 5a-5c and 7a-7d basicallyare eliminated. As far as the permanent magnets of the rotor, the fluxpermeable parts of the stator and the windings are concerned, the motoris symmetric with respect to the plane indicated by the line A--A. Thereis therefore no magnetic force causing a stray magnetic flux across theplane indicated at line A--A. Objects outside the outer stators willtherefore not be affected by varying magnetic fields.

The currents in the two inner stators will flow in different directionsand the voltages which they will induce in the motor shaft willtherefore cancel each other. The same is true for the currents in theouter stator.

The front shield 901 and rear shield 902 can as above be made of amagnetically and electrically nonconducting, thermally conductivematerial like a metal powder composite material or a suitable ceramic.The distance from the exterior surfaces of the shields to the innerstators will reduce the effects or stray flux from inner to outerstator.

FIGS. 9b and 9c show an axial sectional view and radial view of a fourphase rotation motor according to the invention. The motor hasindividually wound poles. This embodiment is basically two motors asshown in FIGS. 1a-1b built together using the same rotational shaft;thus the radial section of FIG. 9c is similar to that of FIG. 1a. Byassembling the two systems 45 electrical degrees apart, the rippletorque can be reduced as compared to a two-phase system.

FIG. 10 shows an embodiment of a linear motor according to the inventionhaving individually wound poles and four phases.

The motor comprises a central slide which is inserted in a slot betweentwo stators. The active parts of the slide comprise 10 magnets1001-1010. These magnets are attached to each other by a magneticallyinactive material, not shown and the kind of which is not relevant tothe invention, in order to form a stiff and straight slide. The magnetpoles may preferably be made of permanent magnet material like NdFeB,NdPrFeB or SnCo. The permanent magnet poles are oriented so that alleven numbered poles have the same polarity and all odd numbered poleshave the opposite polarity. The magnets cover as above somewhat morethan 1/2 of the pole pitch length.

The stator comprises, as viewed in the Figure, one upper and one lowerstator part pair, each pair having 2 phases. The upper stator paircomprises a left part having six poles with a base or support 1063 and 6pole teeth 1021-1026 made of for example laminated iron like in mostconventional electric motors. Depending on the requirements of themotor, most or all poles like 1021-1026 have windings. If the obtainableforce is more important than the winding losses, all poles are wound. Ifthe winding losses are to be kept at a minimum, only those poles whichare always engaged with magnets of the slide are wound.

The motor of FIG. 10 has four phases, each one having or belonging toone pole group; all poles on the top left stator, as seen in the Figure,i.e. the 6 poles 1021-1026, form one group and all poles on the upperright stator, i.e. the 6 poles 1041-1046, form another group.

All stator poles in one group have the same pole pitch, which is thesame as the pitch of the permanent magnet poles on the slide.

The wound poles may thus have a coil wound around each single polesimilar to the winding shown at 123 in FIG. 1b. The coils around theeven numbered poles 1022, 1024 and 1026 are wound in the same directionwhile the coils around the odd numbered poles are wound or connected inthe other, opposite direction according to the same principle as hasbeen described for the motor of FIGS. 1a-1b; the upper stator systemcomprising poles 1021-1026 and 1041-1046 operates in a way similar tothe motor of FIGS. 1a-1b.

The lower stator system is also similar to that of the motor of FIGS.1a-1b, but it has its poles 45 electrical degrees out of phase or offsetin relation to the poles of the upper stator system. The arrangement canbe described as two two-phase motors mechanically connected to eachother having said electrical offset. This will permit a lower ripple ofthe force (less cogging) when the slide is moving and will reduce thereluctance forces.

The poles 1061 and 1062 in the centre of the left and right stator areflux balancing poles and are normally unwound. The purpose thereof is toprovide a high permeability flux path for the permanent magnet polewhich is located between the upper and lower stator parts.

The preferable mechanical range of the slide movement is illustrated bythe arbitrarily selected point 1071 of the slide having the range frompoint 1072 to point 1073.

The motor is illustrated having 6 poles in each group. In many cases thenumber of poles will be much higher. In such cases the number of windingturns for each pole may often be very small.

FIG. 11 shows a winding arrangement for a linear motor according to theinvention similar to that shown in FIG. 10 but intended for motorshaving far more poles in each group. The Figure shows a partial sectionalong the line A--A of the motor in FIG. 10. The winding slot around thepole 1055 is also shown (in the same view as in FIG. 10). The windingcomprises four turns of a bandlike conductor 1301-1304 in the shape oftwo layers both having two conductors like 1303 and 1304 located uponeach other. The slot most close to the flux balancing pole 1062 is notused; in motors with maybe 40 poles in each group the flux loss of onepole like 1051 is negligible. Alternatively, other shapes of the fluxbalancing pole 1062 can reduce this loss even further.

After showing some embodiments of the motors according to the invention,the basic idea of the invention will be described compared toconventional brushless DC motors.

FIGS. 12a-12b show a principal flux pattern in a conventional brushlessDC motor and the motors according to the invention respectively.

FIG. 12a thus shows a section through a conventional brushless linear DCmotor. There are two permanent magnets 1201 and 1202 attached to a backiron 1203 and six stator poles 1211-1216. The section shown correspondsto 360 electrical degrees. There are winding slots 1221-1227, of which1222 and 1225 belong to phase R and slots 1223 and 1226 belongs to phaseS. The dimensions of the system is given in some arbitrary units. Thewinding slots like 1222 are 1 unit wide and 14 units high and have anarea of 13.75 square units, the pole pieces taking 0.25 square unitsaway. The magnets like 1201 are 6 units wide. The flux from the magnetsshould pass the stator poles like 1212. If the magnet 1201 is moved 1.5unit to the right, it will face only stator poles 1212 and 1213. Thepeak flux density in the stator poles will therefore be 1.5 times higherthan in the permanent magnets like 1201 as the flux from 6 length unitsof magnets must pass 4 length units of pole iron.

If the magnets would be moved to the right, the N-flux in stator pole1211 would decrease, the S-flux in stator pole 1214 would decrease, theN-flux in stator pole 1213 would increase and the S-flux in stator pole1216 would increase. Both the R-coil through slots 1222 and 1225 and theS-oil through slots 1223 and 1226 would get more N-flux because of pole1213 and less S-flux because of pole 1214, and both the R and S phasewould get an emf able to create a force on the permanent magnets.

FIG. 12b shows a motor according to the invention. It has a first part(the slide) 1250) consisting of permanent magnet poles 1251 and 1252.Above and below this slide there is one pair of wound stator parts,consisting of an upper stator part 1260 and one lower stator part 1270having poles of magnetically highly permeable material 1261-1263 and1271-1272 facing the two sides of the slide 1250 through two small airgaps 1253 and 1254.

The magnetic orientation of the permanent magnet poles 1251 and 1252 ofthe slide 1250 are basically perpendicular to the two surfaces of theslide that faces the stator parts 1260 and 1270.

For all positions of the slide, most of the permanent magnet poles like1250 of the slide is in a position where it can drive a magnetic fluxloop through the upper airgap 1254, into a facing pole 1262 ofmagnetically highly permeable material of the upper stator part 1260,through this stator part 1260, back through the same upper airgap 1254,through one adjacent permanent slide magnet pole 1252, through thesecond airgap 1253, into a pole 1272 of magnetically highly permeablematerial of the lower stator part 1270, through this lower stator part1270, back through the same second airgap 1253 to the original slidepermanent magnet pole 1251.

In the slide position shown, the position of slide permanent magnet pole1251 is such that this flux loop will pass the winding of the windingslot 1274 of the lower stator part 1270.

As described above for various embodiments like the one shown in FIG.1a, the winding in the slots 1264 and 1265 of the upper stator part 1260belong to only one phase V, and the winding in the slots 1273-1275 ofthe upper stator part 1260 belong to only one phase U.

As has also been described above for various embodiments like the oneshown in FIG. 1a, the emf of the phase (V) of upper stator part 1260 isbasically 90 electrical degrees from the emf of the phase (U) of thelower stator part. In FIG. 12b each stator part has at least two statorpoles facing the slide, and the pitch of the stator poles like 1262 and1263 of a stator part are the same as the pitch of the permanent magnetpoles 1251 and 1252 of the slide facing that stator part 1260. As theFIG. 12b shows a linear motor, the pitch is the same on both sides ofthe slide. In FIG. 1a, illustrating a rotation embodiment, the outerradius of the rotor is larger than the inner radius and the pitches aretherefore different when expressed as a length, but identical whenexpressed as an angle.

The length of the gaps like 1276 between two adjacent stator poles like1271 and 1272 of the lower stator part facing the path of the permanentmagnet poles of the slide is small (0.5 units) compared to the length ofthe stator poles like 1271 and 1272 facing the path of the permanentmagnet poles of the slide (8.5 units).

The stator poles are shaped in such a way that the distance like 1277between the adjacent stator poles like 1271 and 1272 in the same statorpart like 1270 close to the stator part surface facing the path of thepermanent magnet poles of the slide 1250 increases with the distance1278 from the stator part surface facing the path of the permanentmagnet poles of the slide, so that the distance like 1277 in notmagnetically highly permeable material which must be passed by leakageflux is relatively long except very close to the stator part surfacefacing the path of the permanent magnet poles of the slide.

The dimensions of the system is given in the same arbitrary units asused in FIG. 12a. The winding slots like 1274 are 5 units wide and 7units high and have an area of 28.75 square units, the pole piecestaking 6.25 square units away. The magnets like 1251 are 6 units wide.The flux from the magnets should pass the stator poles like 1271. In theposition shown for the magnet 1251 it faces only stator pole 1271 in thelower stator part 1270. The peak flux density in the stator poles willtherefore be 1.5 times higher than in the permanent magnets like 1251,or the same as for the motor of FIG. 12a.

If the magnets would be moved to the right, the S-flux in stator pole1262 would increase for two reasons. In the initial position shown, allthe S-flux into pole 1206 will be taken out of the same pole by theN-flux generated by permanent magnet pole 1252. When the slide moves,more S-flux will enter from magnet 1251, and the same amount of N-fluxwill be withdrawn due to the movement of magnet 1252. Therefore, the Vcoil through slot 1265 will get the same increase of N-flux as the R andS coils in FIG. 12a would have got from the same movement of the slide,and the V phase would get a emf able to create a force on the permanentmagnets.

A comparison between the two motors show that they have approximativelythe same weight and the same volume (both are 20 units high).

The iron losses is given by the volume of iron having high peak to peaktransitions. Assuming that the motors have the same length, this volumeis linear in the area of iron poles and back iron. For the conventionalmotor of FIG. 12a this area is 6×(14×2)+18×2=204 square units. For themotor according to the invention of FIG. 12b this area is2×2×(7×4)+2×2×18=184 square units (two stator part with two poles eachof 7×4 units). The current carrying winding area for the conventionalmotor of FIG. 12a is 2×13.75=27.5 square units (both phase R and S areactive at the same time). For the motor according to the invention asindicated in FIG. 12b this area is 28.75 square units, or almost thesame. This is true for very long motors. For short motors theconventional motor phases require long winding paths outside the iron(as the R and S windings block the way out for the T winding etc), oftenresulting in that less than half the winding length is inside the statoriron. For motors according to the invention, the coils are simply woundaround the pole. There is therefore little need for long wires outsideof the iron stator.

For very low loads, the flux from the permanent magnets will follow theiron poles. At higher loads, the permanent magnet stray flux willincrease. In the motor according to the invention, a stray flux linelike 1290 passing the center of the winding must pass a low permeablepath 5 units long, while the corresponding line 1240 of a conventionalmotor only must pass 2 units length of a low permeable path.

The winding stray flux from the half of the winding distant from thepermanent magnets must pass 1 unit length of a low permeable path in theconventional motor and has a remaining height of 7 units over which tospread its intensity. The stray flux from the half of the windingdistant from the permanent magnets must pass 1-5 units length of a lowpermeable path in the motor according to the invention and has aremaining height of 3.5 units over which to spread its intensity. As theenergy required to build up flux increases linearly with the length andincreases with the square of the intensity, the energy required in theconventional motor is proportional to 1/(7×7) and 3/(3.5×3.5), that is12 times higher, for the motor according to the invention.

Therefore, for motors of the same volume and same pole density, themotors according to the invention will have lower copper losses formotors having a short stator length. They give much lower stray fluxesfor a given current density, which means better peak torques and muchless deterioration of the torque-current motor constant at higher loads.This means that high torques can be obtained with a lower current andthus gives better efficiency and higher continuous torques.

The comparison above assumes equal pole density (both motors have thesame slide pole pitch). The conventional motor as of FIG. 12a can getlower stray flux losses if the pitch is increased. Getting the samewinding slot width of 5 units would require 5 times larger pitch, whichwould increase the height of the backiron and base iron to 10 unitseach, giving no space left for either poles, magnets or windings.

It must be emphasized that the motors according to the invention shouldnot be confused with stepper motors. The shape of the poles and thesmall inter pole gaps close to the rotor and slide make them useless asstepper motors. Referring to FIG. 12b, the slide position will moverather freely 1 unit to the left or right from its shown position ifphase U gets current and phase V is without current. In a stepper motor,the slide or rotor position is well defined when one phase gets fullcurrent.

As is obvious to anyone skilled in the art, the electric motors asdescribed above may undergo changes and modifications within theinventive concepts.

Normal functional details and principles common to brushless motors havenot been mentioned in the descriptions of the principles of operation orof different embodiments. Some examples will be listed here. The motorsrequire some power electronics to control the currents in the windings,and angular transducers for permitting this power electronics toenergize the correct phase(s) with proper currents. The magnets shouldin some cases be divided into electrically insulated sections. Thestators can in some cases be skewed to reduce cogging torque.

The descriptions above have been made for trapezoidal back-emf motorsand for sinusoidal back-emf motors. By changing the shape of individualmagnets, by different magnetization etc., other back-emf form motors canbe obtained. The two basic types described will however permit thoseskilled in the art to make adjustments if necessary for other back-emfforms.

I claim:
 1. A brushless, electric motor or generator comprisinga firstpart comprising permanent magnet poles having equally spaced north andsouth poles and with no or little soft iron parts, at least one pair ofsecond parts, all being rigidly attached to each other, and comprisingelectrical windings and having poles of a magnetically highly permeablematerial, the poles of one member of a second part pair facing one sideof the first part and the poles of the other member of said second partpair facing another side of the first part, an air gap separating themagnetically highly permeable material poles of the second parts fromthe corresponding side of the first part, the first part being movablein relation to the second parts, wherein the magnetic orientation of thepermanent magnet poles of the first part is substantially perpendicularto the two sides of the first part which face the second parts, theshape and distances of the first and second parts are such that, for allpositions of the first part in relation to the second parts, most of thepermanent magnet poles of the first part are in a position where eachpermanent magnet pole can drive a magnetic flux loop through a first oneof said airgaps, into a facing pole of a magnetically highly permeablematerial of a first member of a second part pair, through this firstmember of a second part pair, back through the same first airgap,through one adjacent permanent magnet pole of the first part, through asecond one of said airgaps, into a pole of magnetically highly permeablematerial of the second member of the same second part pair, through thissecond member of the same second part pair and back through the samesecond airgap to the original permanent magnet pole of the first part,for all positions of the first part, most of the permanent magnet polesof the first part are in a position where said flux loops of twoadjacent permanent magnet poles of the first part will pass windings ofat least one or both of the two members of a second part pair, thewinding or windings of one second part belong to only one electricalphase, the emf of the phase of one part in each second part pair issubstantially 90 electrical degrees apart from the emf of the phase ofthe other part in the same second part pair, each second part has atleast two poles facing the first part, the pitch of the poles of eachsecond part is substantially the same as the pitch of the permanentmagnet poles of the first part facing that second part, the area of thegaps between two adjacent poles of a second part and facing a side ofthe first part is less than the area of the poles of said second partfacing the same side of the first part, the poles of the second partsare shaped in such a way that the distance between two adjacent poles inthe same second part and close to the surface of said poles facing aside of the first part increases with the distance from the surface ofsaid poles facing a side of the first part, so that the distance in notmagnetically highly permeable material that must be passed by leakageflux is relatively long except very close to the surface of the secondpart facing a side of the first part.
 2. A motor or generator accordingto claim 1, wherein the poles of the first and second parts are locatedin such a way that those permanent magnet poles of the first part, whichare not in a position facing a pole of a second part, will face a polenot belonging to any second part permitting a short, highly permeableflux path to the adjacent second parts.
 3. A motor or generatoraccording to claim 1 or 2, wherein the first part is a hollow cylinderrotor.
 4. A motor or generator according to claim 1 or 2, wherein thefirst part is a disc-shaped rotor.
 5. A motor or generator according toclaim 1 or 2, wherein the first part is a linear slide.
 6. A motor orgenerator according to claim 1, wherein the poles of the second partshave an area perpendicular to the flux from the first part being asubstantial fraction of the area of the first part magnet pole facingthe second part, this substantial fraction of area of the first partbeing on the order of 0.5 to 1.0.
 7. A motor or generator according toclaim 1, wherein the poles of the second parts have a widthperpendicular to the direction of movement of the first part in relationto the second parts, which is substantially uniform over the length ofthese poles and is substantially the same as the width of the permanentmagnet poles in the same direction.
 8. A motor or generator according toclaim 1,wherein the poles of a second part comprise a first portionlocated at a base member of said second part, said first portion of eachpole having sides, which are located substantially perpendicular to thedirection of movement of the first part in relation to the second partsand also to the surface of the first part adjacent to the pole, that thepoles of the second parts comprise lateral protrusions at their endsfacing the first part, the protrusions being connected to said firstportion and extending substantially in a direction in parallel andantiparallel with the direction of movement of the first part inrelation to the second parts, in such a way that only a small gap in themovement direction is left between adjacent poles of the second partcompared to the extension of said poles of the second part at theirsurfaces facing the first part and in the movement direction.
 9. A motoror generator according to claim 8, wherein the added length of saidlateral protrusions of a pole of a second part in the direction inparallel and antiparallel with the movement direction is about thelength of the first portion of the pole in the same direction.
 10. Abrushless, electric motor or generator comprising:a first partcomprising permanent magnet poles having equally spaced north and southpoles and with no or little soft iron parts, at least one pair of secondparts, all being rigidly attached to each other, and comprisingelectrical windings and having poles of a magnetically highly permeablematerial, the poles of one member of a second part pair facing one sideof the first part and the poles of the other member of said second partpair facing another side of the first part, an air gap separating themagnetically highly permeable material poles of the second parts fromthe corresponding side of the first part, the first part being movablein relation to the second parts, wherein the magnetic orientation of thepermanent magnet poles of the first part is substantially perpendicularto the two sides of the first part which face the second parts, theshape and distances of the first and second parts are such that, for allpositions of the first part in relation to the second parts, most of thepermanent magnet poles of the first part are in a position where eachpermanent magnet pole can drive a magnetic flux loop through a first oneof said airgaps, into a facing pole of a magnetically highly permeablematerial of a first member of a second part pair, through this firstmember of a second part pair, back through the same first airgap,through one adjacent permanent magnet pole of the first part, through asecond one of said airgaps, into a pole of magnetically highly permeablematerial of the second member of the same second part pair, through thissecond member of the same second part pair and back through the samesecond airgap to the original permanent magnet pole of the first part,for all positions of the first part, most of the permanent magnet polesof the first part are in a position where said flux loops of twoadjacent permanent magnet poles of the first part will pass windings ofat least one or both of the two members of a second part pair, thewinding or windings of one second part belong to only one electricalphase, the emf of the phase of one part in each second part pair issubstantially 90 electrical degrees apart from the emf of the phase ofthe other part in the same second part pair, each second part has atleast two poles facing the first part, the pitch of the poles of eachsecond part is substantially the same as the pitch of the permanentmagnet poles of the first part facing that second part, the area of thegaps between two adjacent poles of a second part and facing a side ofthe first part is less than the area of the poles of said second partfacing the same side of the first part, the poles of the second partsare shaped in such a way that the distance between two adjacent poles inthe same second part and close to the surface of said poles facing aside of the first part increases with the distance from the surface ofsaid poles facing a side of the first part, so that the distance in notmagnetically highly permeable material that must be passed by leakageflux is relatively long except very close to the surface of the secondpart facing a side of the first part,wherein in the first part thelength therealong in the movement direction between a first permanentmagnet pole and another magnet pole of the first part is longer than theadded lengths of all magnet poles along the first part from the first tothe other permanent magnet pole, in such a way that a part of the areaof the first part from the first to the other permanent magnet pole isuncovered with permanent magnet poles, the permanent magnet polescovering 55-70% of the length of said area along the first part and inthe movement direction.
 11. A brushless, electric motor or generatorcomprising:a first part comprising permanent magnet poles having equallyspaced north and south poles and with no or little soft iron parts, atleast one pair of second parts, all being rigidly attached to eachother, and comprising electrical windings and having poles of amagnetically highly permeable material, the poles of one member of asecond part pair facing one side of the first part and the poles of theother member of said second part pair facing another side of the firstpart, an air gap separating the magnetically highly permeable materialpoles of the second parts from the corresponding side of the first part,the first part being movable in relation to the second parts, whereinthe magnetic orientation of the permanent magnet poles of the first partis substantially perpendicular to the two sides of the first part whichface the second parts, the shape and distances of the first and secondparts are such that, for all positions of the first part in relation tothe second parts, most of the permanent magnet poles of the first partare in a position where each permanent magnet pole can drive a magneticflux loop through a first one of said airgaps, into a facing pole of amagnetically highly permeable material of a first member of a secondpart pair, through this first member of a second part pair, back throughthe same first airgap, through one adjacent permanent magnet pole of thefirst part, through a second one of said airgaps, into a pole ofmagnetically highly permeable material of the second member of the samesecond part pair, through this second member of the same second partpair and back through the same second airgap to the original permanentmagnet pole of the first part, for all positions of the first part, mostof the permanent magnet poles of the first part are in a position wheresaid flux loops of two adjacent permanent magnet poles of the first partwill pass windings of at least one or both of the two members of asecond part pair, the winding or windings of one second part belong toonly one electrical phase, the emf of the phase of one part in eachsecond part pair is substantially 90 electrical degrees apart from theemf of the phase of the other part in the same second part pair, eachsecond part has at least two poles facing the first part, the pitch ofthe poles of each second part is substantially the same as the pitch ofthe permanent magnet poles of the first part facing that second part,the area of the gaps between two adjacent poles of a second part andfacing a side of the first part is less than the area of the poles ofsaid second part facing the same side of the first part, the poles ofthe second parts are shaped in such a way that the distance between twoadjacent poles in the same second part and close to the surface of saidpoles facing a side of the first part increases with the distance fromthe surface of said poles facing a side of the first part, so that thedistance in not magnetically highly permeable material that must bepassed by leakage flux is relatively long except very close to thesurface of the second part facing a side of the first part,wherein thelength of a permanent magnet pole in the movement direction issubstantially smaller than a length of a portion of each of the poles ofthe second parts, which face a side of the first part, said length ofthe permanent magnet poles being about 55-75% of said length of the poleportions of the second parts and facing the first part.
 12. A brushless,electric motor or generator comprisinga first part comprising a rotor ora slide including permanent magnet poles having equally spaced north andsouth poles, a stator formed from at least one pair of second parts, allbeing rigidly attached to each other and comprising electrical windingsand having poles of a magnetically highly permeable material, the polesof one member of a second part pair facing one side of the first partand the poles of the other member of said second part pair facinganother side of the first part, an air gap separating the magneticallyhighly permeable material poles of the second parts from thecorresponding side of the first part, the first part being movable inrelation to the second parts, wherein a magnetic orientation of thepermanent magnet poles of the first part is substantially perpendicularto the two sides of the first part which face the second parts, theshape and distances of the first and second parts are such that, for atleast some positions of the first part in relation to the second parts,the permanent magnet poles of the first part are in a position whereeach permanent magnet pole can drive a magnetic flux loop through afirst one of said airgaps, into a facing pole of a magnetically highlypermeable material of a first member of a second part pair, through thisfirst member of a second part pair, back through the same first airgap,through one adjacent permanent magnet pole of the first part, through asecond one of said airgaps, into a pole of magnetically highly permeablematerial of the second member of the same second part pair, through thissecond member of the same second part pair and back through the samesecond airgap to the original permanent magnet pole of the first part,for all positions of the first part, most of the permanent magnet polesof the first part are in a position where said flux loops of twoadjacent permanent magnet poles of the first part will pass windings ofat least one or both of the two members of a second part pair, thewinding or windings of one second part belong to only one electricalphase, the emf of the phase of one part in each second part pair issubstantially 90 electrical degrees apart from the emf of the phase ofthe other part in the same second part pair, each second part has atleast two poles facing the first part, the pitch of the poles of eachsecond part is substantially the same as the pitch of the permanentmagnet poles of the first part facing that second part, the area of thegaps between two adjacent poles of a second part and facing a side ofthe first part is less than the area of the poles of said second partfacing the same side of the first part, the poles of the second partsare shaped in such a way that the distance between two adjacent poles inthe same second part and close to the surface of said poles facing aside of the first part increases with the distance from the surface ofsaid poles facing a side of the first part, so that the distance in notmagnetically highly permeable material that must be passed by leakageflux is relatively long except very close to the surface of the secondpart facing a side of the first part.
 13. A motor or generator accordingto claim 12, wherein the first part is a hollow cylinder rotor.
 14. Amotor or generator according to claim 12, wherein the first part is adisc-shaped rotor.
 15. A motor or generator according to claim 12,wherein the first part is a linear slide.
 16. A brushless electric motoror generator comprisinga first part selected from the group consistingof hollow cylinder rotor, a disc-shaped rotor and a linear slide, thefirst part comprising permanent magnet poles having equally spaced northand south poles, a stator formed from at least one pair of second partscomprising electrical windings and having poles of a magnetically highlypermeable material, the poles of one member of a second part pair facingone side of the first part and the poles of the other member of saidsecond part pair facing another side of the first part, an air gapseparating the magnetically highly permeable material poles of thesecond parts from the corresponding side of the first part, the firstpart being movable in relation to the second parts in said air gap,wherein a magnetic orientation of the permanent magnet poles of thefirst part is substantially perpendicular to the two sides of the firstpart which face the second parts, the shape and distances of the firstand second parts are such that, for at least some positions of the firstpart in relation to the second parts, the permanent magnet poles of thefirst part are in a position where each permanent magnet pole can drivea magnetic flux loop through a first one of said airgaps, into a facingpole of a magnetically highly permeable material of a first member of asecond part pair, through this first member of a second part pair, backthrough the same first airgap, through one adjacent permanent magnetpole of the first part, through a second one of said airgaps, into apole of magnetically highly permeable material of the second member ofthe same second part pair, through this second member of the same secondpart pair and back through the same second airgap to the originalpermanent magnet pole of the first part, the permanent magnet poles ofthe first part are in a position where said flux loops of two adjacentpermanent magnet poles of the first part will pass windings of at leastone or both of the two members of a second part pair, the winding orwindings of one second part belong to only one electrical phase, the emfof the phase of one part in each second part pair is substantially 90electrical degrees apart from the emf of the phase of the other part inthe same second part pair, each second part has at least two polesfacing the first part, the pitch of the poles of each second part issubstantially the same as the pitch of the permanent magnet poles of thefirst part facing that second part, the area of the gaps between twoadjacent poles of a second part and facing a side of the first part isless than the area of the poles of said second part facing the same sideof the first part, the poles of the second parts are shaped in such away that the distance between two adjacent poles in the same second partand close to the surface of said poles facing a side of the first partincreases with the distance from the surface of said poles facing a sideof the first part, so that the distance in not magnetically highlypermeable material that must be passed by leakage flux is relativelylong except very close to the surface of the second part facing a sideof the first part.